The link between genes, the environment and depression

Further research is needed to discover all the genetic and environmental factors involved in depression

For many years, researchers have suspected the existence of specific genes that make certain individuals more susceptible to depression than others.

The suspicion arose when it became obvious through observation that some people seem to be more sensitive to stress than others and some develop depression regardless of the events in their lives. There are also many studies indicating depression runs in families.

One study from the University of Wisconsin provided evidence of what that gene might do. The study pinpointed one of the specific genes that appear to make a person more likely to become depressed.

The gene, called 5-HTT, is a serotonin transporter gene and was chosen for study because of the known link between depression and serotonin response to stress. This gene is the blueprint for a specific protein that is critical for transporting serotonin through the neuron cell membrane.

Everyone has the gene 5-HTT, but there are different versions of it. Some people have two short copies, others have two longer copies and some have one of each.

This study found that people with one or two short copies of the gene are more likely to develop depression if they experience multiple stressful events than those with two copies of the protective longer version of the same gene.

More than 800 people were observed from birth into adulthood – 17 per cent carried two copies of the short, stress sensitive gene, while 31 per cent carried two copies of the long protective version and 51 per cent had one copy of each.

During the study, stressful life events were charted when participants were between the ages of 21 and 26. By the age of 26, 17 per cent had experienced major depression in the past year and 3 per cent had contemplated or attempted suicide.

Numbers were significantly higher in the group which had experienced multiple stressful events and also had at least one copy of the short gene – 11 per cent had attempted or contemplated suicide by the time they were 26 years old. Of those who had experienced multiple stresses, and had two copies of the long gene, only 4 per cent had contemplated or attempted suicide.

Clearly, this shows a connection between suicide risk and the gene 5-HTT.

Only 10 per cent of study participants were carriers of at least one copy of the short gene and also experienced four or more stresses, but this group represented 25 per cent of the total depression cases. In fact, 43 per cent of this group developed depression by age 26. Only 17 per cent of those with two copies of the longer gene were depressed.

Although the study showed a connection between gene 5-HTT and risk for depression and suicide, it is clearly not the only factor involved.

The gene’s effects may only be triggered in people who experience stress or trauma in their lives and perhaps not in all of those people. This supports the theory that our environment also plays an important role in determining the course of our mental health.

It is also important to note that while those with the two copies of the longer gene were less likely than others to develop depression, this was not a completely protective gene. Given enough stressful experiences, some of these people still developed depression.

This study was the first to identify a specific gene for depression and since then research has continued. Ongoing research is needed to further examine this link and discover all the genetic and environmental factors involved in depression.

Dr. Latimer is president of Okanagan Clinical Trials and a Kelowna psychiatrist.